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Mathematical Description of Isobutane Alkylation with Butenes in the Presence of Trifluoromethanesulfonic Acid Anatoly S. Berenblyum,*,† Evgeny A. Katsman,‡ and Sven I. Hommeltoft§ Haldor Topsøe A/S (Moscow Representative Office), Bryusov Street 11, 103009 Moscow, Russia, The All Russian Research Institute for Organic Synthesis Company, Radio Street 12, 107005 Moscow, Russia, and Haldor Topsøe A/S, DK-2800 Lyngby, Denmark
Isobutane alkylation with 2-butene kinetics was studied in the presence of trifluoromethanesulfonic acid (TfOH) catalyst. The rate of this reaction is very fast and strongly dependent on acidity. Therefore, the study was conducted in the presence of compounds able to decrease the acidity (acidity controllers) and thereby to decrease the reaction rate to measurable values. The butene surplus and the reaction byproduct, acid-soluble oil (ASO), were used as acidity controllers. The rate of isooctane sum accumulation was studied in the wide variation range of reactant concentrations (from acid surplus to butenes up to a butene surplus) at temperatures from -20 to -45 °C. On the basis of the physicochemical and kinetic studies, the mechanism was suggested for the reaction predominantly proceeding in the acidic phase. The kinetic equations obtained adequately describe the main reaction routes including isooctanes and ASO formation. All route rate equations are first order on the activity of proton in the acidic phase. The effective route activation energies were determined. 1. Introduction The isobutane alkylation process with olefins has been well-known for a long time. The reaction itself was discovered at the beginning of the 1930s by Ipatieff and Grosse.1 On the basis of this discovery, large-scale industrial processes were developed in the 1940s to produce high-octane gasoline components by isobutane alkylation with C3-C5 olefin cuts.2 Recently, data appeared in the literature about this reaction catalysis by triflic acid (trifluoromethanesulfonic acid, TfOH).3 Despite the fact that the main features of the alkylation mechanism using liquid acid catalysts are known,3 there are no data available in the literature on its kinetics in the presence of TfOH. One of the reasons for this is the fact that there are aspects of the alkylation process that complicate a study of the reaction. Among the complicating aspects are the following: (1) The pure TfOH-catalyzed reaction is fast, and it is very difficult to study its kinetics by the usual methods. Two phases (one acidic and one organic) are present in the reaction, and so far it has not been established in which of these phases (or in both) the alkylation reaction proceeds. (2) Acid-soluble oil (ASO), which is formed during the reaction in a considerable amount, is known to have a strong effect on alkylation,2 including the interaction observed between ASO and TfOH catalyst,4 the reaction inhibition due to reduced TfOH acidity,5 and the effect on the TfOH interphase distribution.6 There is no quantitative description available in the literature of the effect of ASO on the alkylation reaction kinetics in the TfOH presence. * To whom correspondence should be addressed. E-mail:
[email protected]. † Haldor Topsøe A/S (Moscow Representative Office). ‡ The All Russian Research Institute for Organic Synthesis Company. § Haldor Topsøe A/S.
In the present work, we succeeded in resolving the above-named problems, and because of that, the kinetics of isobutane alkylation with butenes in the presence of TfOH were studied; on this basis, a mathematical reaction description was developed. 2. Results and Discussion We found that, in the presence of excess pure TfOH (ratio of TfOH/C4H8 g 1), the alkylation reaction proceeded to completion almost immediately even at temperatures of -45 to -20 °C. However, the alkylation rate was strongly dependent on the catalyst acidity.5 If the reaction takes place in the presence of compounds that reduce the acidity (acidity controllers), it is possible to reduce the alkylation rate sufficiently to achieve measurable reaction rates. 2.1. Reaction Kinetics in the Presence of Olefin Surplus. A surplus of n-butenes relative to the triflic acid catalyst acts as a catalyst inhibitor. This effect was studied experimentally at the following conditions: molar ratio of TfOH/C4H8 ranging from 0.75 to 0.95 (in steps of 0.05); molar ratio of C4H10/C4H8 of 4.5, 5.5, 9, and 11. The temperature was -45, -35, and -20 °C. Because ASO was formed in the reaction, analogue experiments were performed in which ASO was initially added to the reaction mixture. The initially added ASO amount was varied in the range from 0 to 5 wt % relative to the acid. Each experiment was performed according to the following experimental procedure. A solution of triflate ester was prepared by the addition of an excess of 2-butenes to frozen TfOH at -60 °C (the molar ratio of TfOH/2-butenes was 0.75-0.95). The temperature was subsequently increased to -50 °C for 1-2 h. In this way, a homogeneous solution was obtained that, according to the 1H NMR data contained secondary butyl ester [chemical shifts in ppm: 1.0 (triplet, 3H, CH3), 1.46 (doublet, 3H, CH3), 1.7 (multiplet, 2H, CH2), 5.0 (sextet, 1H, CH)] and unreacted butenes. In the presence of a butene surplus, this sec-
10.1021/ie040032l CCC: $27.50 © 2004 American Chemical Society Published on Web 09/25/2004
Ind. Eng. Chem. Res., Vol. 43, No. 22, 2004 6989 Table 1.
a
A. Possible Mechanism Steps of Isobutane Alkylation with Butenes in the Presence of TfOH C4H8 + TfOH S C4H9+‚TfO1 + C4H9 ‚TfO S TfOC4H9 2 TfOC4H9 + H+ S C4H9+‚TfOH 3 C4H9+‚TfOH S TfOH + C4H9+ 4 C4H9+‚TfOH + TfOC4H9 S C8H17+‚TfOH + TfOH 5a + + C4H9 + TfOC4H9 S C8H17 + TfOH 5b C4H9+‚TfOH + C4H8 S C8H17+‚TfOH 5c C4H9+ + C4H8 S C8H17+ 5d C8H17+‚TfOH + C4H10 S C4H9+‚TfOH + C8H18 6a C8H17+ + C4H10 S C4H9+ + C8H18 6b C4H10 + C4H8 S C8H18
Reaction Route
1+2+5+6
B. Unsaturated Oligomers and ASO Formation Routes 7TfOC4H9 S C28H56 + 7TfOH 7a 7C4H8 S C28H56 7b C28H56 + 2TfOH w ASO‚2TfOHb + 2C4H10 8 C. Complex Formation Reaction and Interphase Distribution of It and TfOH TfOHap S TfOHhp 9 ASO‚2TfOHap S ASO‚2TfOHhp 10 ASOhp + 2TfOHhp S ASO‚2TfOHhp 11 a The subscripts ap and hp identify the acid and hydrocarbon phases, respectively. Protons and carbenium cations are supposed to be solvated by TfOH and ASO‚2TfOH b According to data given in ref 4, ASO roughly has the chemical composition C20H36.
butyl triflate ester was stable for short periods of time even at room temperature. Even though the NMR data showed that the ester was thermodynamically stable, it is kinetically labile. This can be illustrated by the fact that the addition of propylene into such a sec-butyl triflate solution results in the formation of isopropyl triflate [1H NMR chemical shifts: 1.45 ppm (doublet, 6H, CH3), 5.25 ppm (septet, 1H, CH)] and in a considerable increase of the free butene concentration. The reversibility of the ester formation reaction is also illustrated by the fact that the addition of the other acidity controller, triethylammonium triflate,5 to the system results in a concentration increase of free butenes (observed by 1H NMR). In both cases, acid is removed from the equilibrium: TfOH + n-butene S sec-butyl triflate (reaction 1 + 2 in Table 1), shifting the equilibrium to the left and causing liberation of butene. Once the homogeneous sec-butyl triflate solution was obtained, liquid isobutane was added to this solution, and the alkylation reaction was observed at temperatures of -45 to -20 °C. For a period of time, the system was homogeneous and gradually changed color from colorless to yellow. During this period, gas chromatograph (GC) analysis data showed that almost no C8 hydrocarbons were formed, but a decrease in the concentration of free olefins was observed at the same time as heavy products were formed (oligomers and ASO). After an initial period with relatively small changes in composition, a rapidly accelerating C8 hydrocarbon formation was observed and a new (acidic) phase formed. If the agitation is stopped at this point, the system will be separated into a yellow-brown (due to ASO formation) acidic phase and a colorless hydrocarbon phase containing no free butenes. Thus, the alkylation reaction proceeds quickly once the acidic phase is formed. The time from the start of the reaction until the alkylation reaction accelerates (Figure 1) may be perceived as an induction period (τ), and its duration
depends on the reaction conditions. The induction period decreases with increasing temperature, increasing acid strength, increasing acid-to-olefin ratio, and increasing isobutane-to-olefin ratio. The observed accelerated C8 formation is evidence of an autocatalytic alkylation mechanism in which free acid liberated in the course of the reaction catalyzes the reaction. During the induction period, the presence of surplus olefin keeps the acid activity low, and consequently the alkylation reaction is strongly inhibited. However, the concentration of free butenes decreases because of their conversion into heavy products. When the olefin surplus is gone, the alkylation rate picks up sharply. If ASO is added to the acid from the start of the experiment and the initial acid activity of the system in this way is lowered, the length of the induction period will increase dramatically (Figure 1). The main reaction products were C8 isoparaffins and ASO. Other paraffins were not formed in noticeable amounts. The C8 isoparaffins predominantly consisted of the four different trimethylpentane isomers. 2.2. Reaction Kinetics in the Presence of an Acid Surplus Containing Considerable ASO Concentrations. As indicated above, the alkylation reaction normally proceeds very fast when a surplus of acid relative to olefin is present. However, it is possible to slow the reaction rate through addition of ASO, which lowers the acidity to a level at which the alkylation reaction is sufficiently slow to be monitored by simple GC methods. Because experiments carried out to study this effect were performed without an olefin surplus, no induction period was observed (Figure 2). The slowing down of the reaction by ASO addition was exploited to study the reaction in the presence of surplus acid relative to olefin at the following conditions: molar ratio of TfOH/C4H8 of 5, 10, and 20; molar ratio of C4H10/C4H8 of 11, 22, and 55. The molar fraction of ASO in a TfOH solution varied from 0.13 to 0.16 in steps of 0.0075. The temperature was -30 and -20 °C. 2.3. Mathematical Description of Reaction Kinetics. By using butene and ASO as the reaction rate controllers, it was possible for the first time to study the kinetics of isobutane alkylation and associated reactions both in the one-phase system (the hydrocarbon phase in the induction period) and in the twophase acid/hydrocarbon system. This study as well as literature data7 give reason to believe that alkylation predominantly proceeds in the acidic phase. Besides, as has been shown previously,5 the rate of isobutane alkylation is related to Hammett’s acidity function. The reaction includes a quick reversible interaction of olefin with acid, forming ester most likely through steps 1 and 2.7 As mentioned above, sec-butyl triflate is a relatively stable compound when a surplus of olefin keeps the acid activity low. The fact that it is possible with a small surplus of olefin to convert triflic acid into ester on a quantitative basis without the alkylation reaction proceeding suggests that the rate of 1 + 2 is sufficiently high to keep the acid activity low at least at the conditions used for the preparation of the ester solution. This is consistent with the previously reported notion to the effect that isobutane alkylation proceeds in two stages. The first stage (steps 1 + 2 in Table 1) is the formation of esters, and these esters alkylate isobutane in the second stage provided that the acid activity is sufficiently high.7
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Figure 1. Experimental values and those calculated by model curves of isooctane cumulation at -20 °C.
When the acid activity is high, the ester reacts immediately. It seems reasonable to assume that the ester is activated by protonation (step 3 in Table 1). The cation formed, C4H9+‚TfOH, may or may not dissociate before further reaction according to step 4. Step 3 may be perceived as a chain-initiating step for a chain reaction consisting of steps 5a + 6a in which sec-butyl ester reacts with isobutane in an acid-catalyzed reaction, forming alkylate and acid. The results obtained in the present study do not rule out the involvement of steps 5b-d or 6b. However, a direct involvement of olefin (steps 5c and 5d) seems to be unlikely when considering that the reaction rate increases with increasing acid activity, which results in a reduction in the olefin concentration through reaction 1 + 2. For those experiments for which an induction period has been observed due to surplus olefin, it seems as if only a small amount of C8 alkylate is formed during the induction period, and the surplus olefin seems to be converted into heavier hydrocarbons and ASO. This ASO formation may proceed through degradation of the ester according to eqs 7a and 8. On the other hand, previous work4 has shown that butenes can be converted into heavy hydrocarbons, and direct oligomerization of excess olefin to form ASO (steps 7b and 8) cannot be ruled out because the concentration of olefins is relatively high during the induction period. It is known that TfOH and ASO are involved in three equilibriums (steps 9-11), affecting mainly the system phase state.6 Equations 1-11 form an initial base for the development of a range of different mathematical descriptions of the reaction kinetics. Obviously, while the alkylation reaction can be represented by elementary steps (steps 1-6 in Table 1), the unsaturated oligomers
and ASO formation reactions are very much complicated, and so they must be treated in a form of summary stoichiometric reactions (reaction routes) 7 and 8. To formulate kinetic model hypotheses, the following main alternatives and their combinations were used and then analyzed: (1) different kinetic equation types, kinetic orders, and chemical equation stoichiometry; (2) the possibility of slow or fast, reversible or irreversible, catalyzed and/or uncatalyzed stage proceedings; (3) the description of catalytic activity in the acidic phase in terms of the concentration or activity of acid, proton (according to Hammett’s acidity function), carbenium ions, and/or ASO‚2TfOH complex concentration. For every formulated hypothesis (in all about 200) the numeric computer treatment was performed using previously described methods.8-12 The computer analysis suggests the following: (1) The carbenium ions in the reaction path form a complex with a TfOH molecule. (2) Hammett’s acidity function is sufficient to predict the overall catalyst activity. (3) It is not possible to determine unambiguously whether the actual alkylation reaction 5 involves only olefin, only ester, or both (5a-d); a satisfactory mathematical description is obtainable for any of these assumptions. (4) ASO is formed indirectly through intermediate formation of what we assume is unsaturated oligomers (steps 7a,b and 8). Let us note that more than one hypothesis gives an adequate description of the experimental data. Therefore, a description that is characterized by simpleness and convenience for engineering calculations as well as by uniqueness of rate constants and kinetic order valuations was chosen. The model chosen includes eqs 1, 2, 5a, 6a, 7a, and 8-11.
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Figure 2. Experimental values and those calculated by model curves of isooctane cumulation: part m at -30 °C and the rest at -20 °C. Table 2. Kinetic Equations for the Alkylation Reaction and Its Parameter Values at a Temperature of -20 °Ca-g reaction route 1 + 2 + 5a + 6a 7 8
reaction rate, mol/min 7.7 × 3.9 × 9.9 ×
10-14[C
0.3 4H8] [C4H10]aH+Vap 10-14[C4H8]0.4aH+Vap 10-13[C28H56]aH+Vap
relative error, %
Ea, kcal/mol
12 17 25
14.7 17.4 4.3
a The route 1 + 2 equilibrium constant value amounts to 25 L/mol. b Hammett’s acidity function is calculated by the equation -H ) 0 14.1 - 13.28NASO + 62.12NASO2 - (2.887 + 0.123t)[C4H8], where H0 ) -log aH+ and t denotes the temperature in °C. c NASO denotes the d molar fraction of ASO in a mixture of ASO and TfOH. [C4H8] in kinetic equations denotes the total concentration of free olefin and olefin present in the form of an ester. e The olefin, ester, isooctane, and oligomer concentrations are difficult to determine in an acidic phase. Thus, kinetic model equations use the corresponding component concentrations in a hydrocarbon phase while assuming equilibrium between the acidic and hydrocarbon phases. f Vap denotes the acidic phase volume. At great acid to olefin excess (>10), Vap approximates by the TfOH and ASO volume summation. The exact value of Vap is obtained during determination of the system equilibrium composition described by eqs 9-11 and 1 + 2. g Reactant concentrations are expressed in moles per liter, time in minutes, and volume in liters.
In Table 2, the corresponding rate equations for reaction routes 1 + 2 + 5a + 6a, 7a, 8 are presented. Parameters of equilibrium steps 9-11 are described in ref 6. The overall isobutane alkylation is represented by reaction route 1 + 2 + 5a + 6a (Table 2). Although this reaction is reversible, it is strongly shifted to the right at the experimental conditions used in this study, and the rate is therefore adequately described as the rate of an irreversible reaction. The same concerns the reaction 7 rate. The reaction rate depends on olefin and
isobutane concentrations as well as on acid activity and volume. Even more complicated is the oligomer formation reaction from butenes (reaction 7), which may also offer a partial explanation for the fractional kinetic order of the butene concentration. As for reaction 1+ 2 + 5a + 6a, rates of reactions 7 and 8 depend on the proton activity and the acidic phase volume. The proton activity is determined from Hammett’s acidity function, H0, which has previously been described for the TfOH/ ASO system.5 It has been proven to be necessary to
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Table 3. Alkylation Rate Dependence on Temperature at an Initial TfOH:C4H8 ) 10:1, C4H10:C4H8 ) 10:1, and a Volume Ratio of Hydrocarbon to Acidic Phases of 1:1 (the Time τ To Obtain 99% Olefin Conversion) no.
temp, °C
τ × 103, s
no.
temp, °C
τ × 103, s
1 2 3
-20 0 10
22 4.3 2.5
4 5
20 30
1.7 1.3
include a contribution from the olefin concentration to the polynom used to estimate H0 (see footnote a, Table 2). The study of the alkylation rate temperature dependence showed that the reaction rate increases moderately with temperature. Because it is difficult to obtain experimental data for the temperature dependence of equilibrium parameters for the reactions 1 + 2 and 9-11 as well as for the acidity function, we have assumed that the temperature dependence for these equilibria can be ignored. This has allowed us to obtain values for “effective” activation energies for the routes 1 + 2 + 5a + 6a, 7a, 8. The kinetic model presented in Table 2 assumes the presence of a separate acidic phase. However, in those experiments where the initial reaction mixture contains an n-butene surplus relative to TfOH, the reaction proceeds in a homogeneous system without a separate acidic phase (all components are in the organic phase). Because the acidity is not well described in the organic phase, the form and physical meanings of the alkylation kinetics in this organic phase are difficult to discuss. The data obtained in the experiments with an initial olefin surplus with respect to acid (Figure 1) are consistent with the mathematical model discussed above from the moment when 1-2 vol % of the acidic phase is formed as a result of the reaction. In Figure 1, the fit between the experimental data and those calculated by the model is shown. The model also predicts simultaneousness between the sharp acceleration reaction period and the appearance and accumulation of the acidic phase. This is consistent with a reaction proceeding mainly in the acidic phase. In the following, the mathematical model is used to estimate reaction rates for isobutane alkylation at typical conditions of the real alkylation process. See Table 3. The data obtained show that the reaction in the temperature range of -20 to +30 °C is very fast, which illustrates why it is difficult to measure the reaction rate directly. 3. Experimental Part A 99% pure triflic acid containing no more than 0.1% water was used without preliminary purification. The isobutane was 99.6% pure, the butenes contained 81.5% 2-butenes (cis-to-trans isomer ratio of 1:1.3) and 18.0% n-butane. At conditions of the present study, both isomers have similar reactivities in observed reactions. ASO was prepared according to the procedure described in ref 4. The moisture content of the hydrocarbons did not exceed 100 ppm. The reaction products were analyzed by gas-liquid chromatography with a flame ionization detector. For GC analysis of the hydrocarbons, capillary quartz columns with a length of 50 m and an internal diameter of 0.2 mm with liquid-phase OV-101 were used. Analysis samples of 0.3 mL were taken by a cooled syringe,
placed in a cooled hermetic sampler, and kept before the analysis at -78 °C on dry ice. A microsyringe cooled on dry ice was used for injection into the GC. The products were identified using a GC-mass spectrometer (MS) of the type Automass-150, UNICAM Mass Spectrometry Group with electron impact ionization (70 eV) and a chromatographic column with a length of 25 m and a diameter of 0.25 mm (liquid-phase OV-1). sec-Butyl ester of triflic acid was identified by the 1H NMR spectrum collected at temperatures of -50 and -10 °C in the presence of a small amount of butene. Similar spectra were registered after 10-fold dilution with isobutane or deuteriochloroform (Fourier spectrometer Bruker, AC-P 300). Similar 1H NMR spectra were collected for the organic phase in equilibrium with solutions of the 0.15-0.20 molar fraction of ASO in TfOH in the presence of a deficiency of butene. Kinetic experiments were performed using a turbinetype stirrer at a speed of 3000 rpm, which proved sufficient to practically avoid transport limitations on the reaction rates. From ∼1500 rpm and higher, there was practically no effect of the agitation level on reaction rates. Kinetic runs were repeated two to three times using different reagent batches (especially TfOH). The measurement reproducibility was about (10%. 3.1. Procedure for Performing Kinetic Runs in the Presence of a Butene Surplus with Respect to TfOH. The kinetic runs applying a butene surplus relative to acid were performed by the following procedure. The necessary amount of acid or an ASO solution in acid was added in a dry argon atmosphere to a three-necked glass reactor cooled to -60 °C equipped with a thermometer and stirrer. Once the acidic phase was frozen, butenes cooled to -78 °C were added and the stirrer was switched on. The reactor temperature was kept at -50 °C, using a cooling bath. After dissolution of all of the solid acid, isobutane cooled to -78 °C was added, and the homogeneous system obtained was quickly heated to the reaction temperature. Each run took 0.5-5 h with some short stops for sampling. 3.2. Procedure for Performing Kinetic Runs Using a Surplus of TfOH Containing Dissolved ASO. The kinetic runs with acid surplus were performed by the following procedure. An ASO solution in acid in the prescribed concentration was added in a dry argon atmosphere to a three-necked glass reactor cooled to -40 °C equipped with a thermometer and stirrer. Isobutane cooled to the same temperature was added, and the stirrer was switched on. Once the temperature was stable, butenes were introduced, and the liquid twophase system obtained was quickly heated to the reaction temperature. The run took about 1 h with short stops for sampling. Acknowledgment The authors thank Ludmila Ovsyannikova, Emma Zhlobich, and Yury Karasev for their help with the experimental work. Literature Cited (1) Ipatieff, V. N.; Grosse, A. V. Reaction of paraffins with olefins. J. Am. Chem. Soc. 1935, 57, 1616. (2) Albright, L. F. Updating alkylate gasoline technology. CHEMTECH 1998, 28 (6), 40.
Ind. Eng. Chem. Res., Vol. 43, No. 22, 2004 6993 (3) Hommeltoft, S. I. Isobutane alkylation: Recent developments and future perspectives. Appl. Catal. A 2001, 221, 421. (4) Berenblyum, A. S.; Ovsyannikova, L. V.; Katsman, E. A.; Zavilla, J.; Hommeltoft, S. I.; Karasev, Yu. Z. Acid soluble oil, byproduct formed in isobutane alkylation with alkene in the presence of trifluoro methane sulfonic acid Part I Acid soluble oil composition and its poisoning effect. Appl. Catal. A 2002, 232, 51. (5) Katsman, E. A.; Berenblyum, A. S.; Zavilla, J.; Hommeltoft, S. I. Poisoning effect of acid soluble oil on triflic acid catalyzed isobutane alkylation. Kinet. Catal. 2004, in press. (6) Katsman, E. A.; Berenblyum, A. S.; Zavilla, J.; Hommeltoft, S. I. Interphase distribution of triflic acid and acid-soluble oil in the isobutane alkylation with olefins. Kinet. Catal. 2003, 44 (6), 757. (7) Hommeltoft, S. I.; Ekelund, O.; Zavilla, J. Role of ester intermediates in isobutane alkylation and its consequences for the choice of catalyst system. Ind. Eng. Chem. Res. 1997, 36 (9), 3491. (8) Akhmetov, N. G.; Katsman, E. A.; Malyugina, S. G.; Mstislavsky, V. I.; Oprunenko, Yu. F.; Roznyatovsky, V. A.;
Ustynyuk, Yu. A.; Batsanov, A. S.; Ustynyuk, N. A. Tricarbonylchromiurn complexes with phenalene. Synthesis, structure and thermal rearrangements. Russ. Chem. Bull. 1997, 46 (9), 1769. (9) Gorsky, V. G. Designing of Kinetic Experiments; Nauka: Moscow, 1984 (in Russian). (10) Brandt, S. Statistical and computational methods in data analysis; North-Holland: Amsterdam, The Netherlands, 1970. (11) Gorskii, V. G.; Katsman, E. A.; Klebanova, F. D.; Grigor’ev, A. A. Numerical study of parameter identifiability for nonlinear models. Teor. Eksp. Khim. 1987, 23 (2), 181. (12) Gorsky, V. G. A prior parameter identifiability analysis of fixed structure models. In Design of experiments and data analysis: new trends and results; Letzky, E. K., Ed.; ANTAL: Moscow, 1993.
Received for review January 26, 2004 Revised manuscript received August 10, 2004 Accepted August 10, 2004 IE040032L
